dt-bindings: iio: update STM32 timers clock names

Clock name has been updated during driver/DT binding review:
https://lkml.org/lkml/2016/12/13/718


Update DT binding doc to reflect this.

Fixes: bf2d8581 (iio: Add bindings for STM32 timer trigger driver)
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>